Charged-particle optical systems are well known, highly specialized tools for illuminating a specimen with charged particles, e.g., electrons or ions in order to create an image or to modify a surface of the specimen. Different configurations of such tools are referred to by their respective names in the jargon, such as Transmission Electron Microscopes (TEMs), Scanning Electron Microscopes (SEM), Reflection Electron Microscopes (REMs), Scanning Transmission Electron Microscopes (STEMs), Focused Ion Beam apparatus, etc. The compartment wherein the specimen is being illuminated is kept evacuated to a sufficiently low pressure so as to prevent the charged particles from being scattered. Electromagnetic or electrostatic lenses are used to control and collimate the charged-particle beam. The specimen is placed, typically accommodated on a carrier, on a specimen holder that extends through the wall of the charged-particle optical system in order to hold the specimen at the required position with respect to the beam. The specimen holder has an end at which the specimen is located, referred to as the “specimen holder tip”.
Some charged-particle optical systems have specimen holders that enable to vary, in situ, the position of the tip and/or the orientation of the tip with respect to the stationary holder so as to be able to make a series of images of the specimen, e.g., for a three-dimensional model of the specimen. The specimen is mounted on the tip when the holder is removed from the charged-particle optical system. For example, U.S. Pat. No. 7,291,847, incorporated herein by reference, discloses a TEM with a specimen tip holder assembly for mounting a specimen tip in the TEM. The specimen tip holder assembly comprises a tip holder for supporting a specimen tip. The tip holder is coupled to an elongate support for movement in a direction substantially perpendicular to the axis of the support. An actuator is mounted to the support for causing motion of the tip holder relative to the support. By means of bonding the specimen to the specimen tip rather than using clips or other mechanical fixing means, the size of the specimen tip may be significantly reduced. Indeed, the need for any sort of specimen clamping or locating mechanism is eliminated thus allowing a much thinner tip profile to fit between the narrow gap of the TEM pole pieces. Once bonded, the individual specimens can be handled and stored as an assembly with an interchangeable specimen tip.
Yet other charged-particle optical systems have been specially designed for cryogenic applications, e.g., the study of biological specimens. In such a system the specimen to be studied is cooled down to the temperature of, e.g., liquid nitrogen or liquid helium and mounted at the specimen holder tip.
In order to keep the specimen at the required low temperature, one configuration of such a system has a cooling device mounted at the specimen holder and in thermal contact with the specimen. This configuration, however, has the disadvantage that the entire specimen holder has to be removed from the system for replacing the specimen at the specimen holder tip. The replacement of the specimen may not be carried out at the same low temperature during operational use. As a result, upon reintroducing the specimen holder with the new specimen into the system, the entire system is not in thermal equilibrium, giving rise to thermal drift of the specimen relative to the electron beam, adversely affecting the image quality. Accordingly, the system has to reach thermal equilibrium before high-quality images can be made, thus reducing the time wherein the system is available for operational use.
U.S. Pat. No. 5,986,270, incorporated herein by reference, discloses a configuration of an electron microscope for cryogenic applications. Herein, the specimen holder is retained in a fixed position in the evacuated compartment during normal operation of the electron microscope. The specimen holder has a control unit, accessible from outside the electron microscope, for translating and rotating the specimen holder. The specimen is brought to the holder via a loading/unloading unit that is mounted separately from the specimen holder and that is configured for transporting the specimen to and from the specimen holder. The loading/unloading unit is mounted in a wall of the electron microscope in the vicinity of the location where the specimen holder is arranged, i.e. preferably at the same height in the wall of the column of the electron microscope and diametrically opposite the specimen holder. The object space accommodating the specimen in operational use of the electron microscope also accommodates a part of a loading/unloading unit. The location of the loading/unloading unit offers the advantage that it is not necessary to modify the design of the apparatus, notably the optical aspects thereof, because the location of the optical elements in the particle-optical column is not influenced thereby. The loading/unloading unit transfers the specimen into and out of the evacuated space within the column of the electron microscope and transports the specimen to and from the specimen holder within said space. The specimen itself need not yet have reached the desired final temperature, because it consists of only a small quantity of matter to be cooled, so that it has a small heat capacity and can be readily cooled, not causing noticeable heating of the specimen holder. The specimen to be studied is introduced into the loading/unloading unit from, after which the specimen is transferred into the object space in a known manner (for example, as in the case of conventional specimen holders); it is then situated at an end of an arm of the loading/unloading unit. The end of the specimen holder is subsequently brought into contact with the end of the arm of the loading/unloading unit in order to take over the specimen. During the transfer of the specimen from the loading/unloading unit to the specimen holder the thermal contact between the end of the specimen holder and the corresponding end of the loading/unloading unit can be limited by providing thermal insulation of a transfer arm of the loading/unloading unit and/or by constructing this arm to be thin and/or by limiting the duration of the contact. It will thus be evident that the specimen holder itself is not removed from the apparatus in order to exchange the specimen. Actually, the specimen holder cannot be removed during normal operation of the system without major servicing of the system. The cooling conduit means are provided separately between the end of the specimen holder which is situated inside the apparatus and the cold source. The term “separately” in this context is to be understood to mean that the cooling conduit means is not connected to the specimen holder in any way other than the connection to the end of the specimen holder. As a result, cooling takes place where cooling is indeed most desirable, i.e. in the direct vicinity of the specimen.
US patent application publication 2008/0250881, incorporated herein by reference, relates to a specimen holder suitable for being used with a replaceable specimen carrier. More specifically, US patent application publication 2008/0250881 relates to a composite structure of a specimen carrier and a specimen holder. The specimen carrier is separately embodied from the sample holder. The specimen carrier can be formed from a strip of metal, and is a simple and inexpensive element. Using resilient force, it clamps onto or into the specimen holder. The portion of the specimen holder to which the specimen carrier couples also has a simple form. The specimen carrier can couple to the specimen holder in vacuum using a coupling tool. In an embodiment of the co-operative composite structure the coupling tool is an automatic coupling tool. An automatic coupling tool makes it possible to automate processes, and it also makes it possible to attach the sample carrier to a sample holder in vacuum. The latter is of particular importance in so-called cryogenic applications, where a sample carrier that has already been pre-cooled has to be attached to a sample holder at cryogenic temperatures, e.g. temperatures in the vicinity of that of liquid nitrogen or of liquid helium. Systems configured for use with replaceable specimen carriers are also highly relevant for (automated) processing of a batch of specimens, each accommodated on a different carrier.
In a further embodiment, coupling of the specimen carrier to the specimen holder or decoupling of the specimen carrier from the specimen holder occurs in vacuum. The vacuum may be part of a TEM, a STEM, a SEM, an Electron Microprobe Analyzer (EPMA), a Focused Ion Beam apparatus (FIB), an Auger analyzer, a Secondary Ion Mass Spectrometer (SIMS), a Scanning Probe Microscope (SPM), an X-ray analyzer, a sputter coater, a plasma cleaner or an evaporative deposition unit.